I don't think any e-readers are designed to be user-replaceable except the very first Kindle. As for DIY replacement, I don't really know, sorry.
> A replacement battery manufactured shortly after the reader is going to be as bad as the one that came installed by the time I need it.
Is it? I was under impression that battery mostly degrade with cycle count. At least, there are plenty of people still use Kindle from 10 years ago.
> [...] warranty replacement is a solution.
Then I think you are mostly limited to Kindle or Kobo. Most other e-reader or e-ink tablet are mostly shipped from China so warranty and replacement are going to not be as good.
Kindle Paperwhite (2021) is one of the fastest E-reader available right now (in term of responsive ness). Kindle device have excellent dictionary too, especially for non-latin language. (Japanese dictionary implementation on Kobo is not very good).
Kobo is more open and is price competitively with Kindle. It's also much more customizable if you are into that.
There are also a lot of Android e-ink tablet like BOOX. These are more expensive but a much, much more customizable as it's basically an Android reader.
Contrast shouldn't be a problem with any recent device. But anything release in past 1-2 years are excellent.
IIRC, the price of 2 year subscription is still cheaper than the 1-year update of the old model. They even offered subscription discount when they were transitioning, so the out-of-pocket is still cheaper than the old model.